There are known stereo image processing apparatuses that compute image shifts based on two images (a target image and a reference image) of an object taken by a stereo camera, and compute the distance to the object based on that shifts. Applying of stereo image processing apparatuses to vehicle-mounted apparatuses is being considered, where the distance to a vehicle or a pedestrian is measured based on stereo images including the vehicle or the pedestrian, for example. However, due to the reduction in the sizes of cameras (e.g., vehicle-mounted cameras) in recent years, camera separations are also becoming smaller, as a result of which disparities between stereo images are also becoming smaller. Accordingly, accurate disparity computation functionality is beginning to be demanded of stereo image processing apparatuses.
As an accurate stereo matching (disparity computation in stereo image processing) scheme, a one-dimensional phase-only correlation (POC) scheme has been proposed, for example (see Patent Literature 1, for example). In a one-dimensional POC scheme, partial images (one-dimensional image data sequences) are extracted from a target image and a reference image using the Hann window. The extracted partial target image and partial reference image undergo a one-dimensional Fourier transform, and are combined. The combined image data has its amplitude components normalized, and undergoes a one-dimensional inverse Fourier transform. Thus, phase-only correlation coefficients are computed, and the shift between the images (i.e., the disparity) is computed based on a correlation peak of the phase-only correlation coefficients.